In a bowl, stir together the Dijon, soy sauce, vegetable oil, brown sugar, minced garlic, and some freshly cracked pepper. Add the chicken thighs to a large zip top bag or shallow dish, and then pour the prepared marinade over top. Refrigerate the marinating chicken for at least 30 minutes (can be prepared in the morning and refrigerated all day).
When you’re ready to cook, begin to preheat the oven to 375 degrees. Peel and cube the sweet potato into ¾ inch pieces. Lay the cubed sweet potato in the bottom of a casserole dish. Lay the chicken thighs over the sweet potatoes and pour all of the marinade over top.
Cover the dish with foil and roast in the preheated oven for 45 minutes, removing the foil after the first 30 minutes. After baking, spoon some of the liquid from the bottom of the dish over the chicken and serve. Use the extra juices from the bottom of the dish to spoon over rice and vegetables.